In building temperature and humidity control systems, a typical arrangement includes one or more compressors for supplying a pressurized pneumatic control fluid, usually air, to a pneumatic control bus disposed within the building walls. Controlling devices such as thermostats and humidistats are coupled to this control bus and to remotely located equipment such as furnaces, air conditioners or humidifiers for regulating and maintaining a particular condition within a space such as a building room. In order to avoid undesirable moisture condensation within the lines or tubing comprising the pneumatic control bus, the pressurized fluid discharged from the compressor is often dehumidified to a relatively low humidity level, typically only a few percent.
Since control devices such as humidistats are coupled directly to this control bus and since such humidistats modulate the control bus pressure for generating a control output signal, it is apparent that any relatively dry pneumatic control fluid which is allowed to contact the humidity sensing element will cause an incorrect control signal to be generated. Specifically, the element will incorrectly sense that the surrounding ambient air has a substantially lower humidity level than exists, in fact. While the problem is lessened to a degree by substantial air movement within the room which may naturally aspirate the humidistat, it may become more acute in sealed rooms with little or no air movement and particularly so where the level of relative humidity of the fluid in the pneumatic control bus is very low compared to that of the room ambient air.
One approach to the solution of aspirating devices such as thermostats and humidistats is embodied in humidistat transmitter model H-52-112 available from Robertshaw Controls Co. This device incorporates a flexible rubber boot for surrounding and covering the control nozzle. The boot incorporates a movable diaphragm portion having a pin molded thereinto and movable therewith for positionably modulating a steel sphere to regulate the effective area of the nozzle orifice. An exhaust passage is in fluid communication with the space between the boot and the nozzle for flowing dry control air to the device exterior. The presence of the diaphragm introduces a spurious, spring-like force which may impair the accuracy of control.
Another approach is shown in U.S. Pat. No. 3,232,112 which illustrates an aspirator jet nozzle disposed upstream, separate from and in parallel with the modulated control port. The nozzle directs a jet of air toward an elongate aperture or slot which creates a Venturi effect to force air out of certain exhaust slots, thereby drawing ambient air inwardly through other intake slots.
Yet another approach is shown in U.S. Pat. No. 3,998,384 which shows an aspirator member disposed on the upstream side of the control port. The member includes a housing having a plurality of shaped chambers and an output jet for directing air toward a rectangular slot in the rear of the apparatus housing. Ambient air is thereby drawn by Venturi action into the housing through intake slots.
Still another approach is shown in U.S. Pat. No. 4,141,496 which uses an aspirator pipe upstream from and in parallel with the device control port. A jet of air is directed through a nozzle and across an aperture for the purpose of drawing ambient air into the device housing by Venturi action.
While these devices have heretofore been satisfactory, they tend to be characterized by certain disadvantages. In particular, those described in the aforementioned patents employ a separate source of aspirating air for effecting a circulation of ambient air to maintain device accuracy. Since these sources of aspirating air represent an additional load demand for the building compressors, they have adverse implications for compressor sizing and/or for the number of installed pneumatic buses. An apparatus which incorporates a discharge element in series with and downstream from the control port, which avoids the introduction of possibly-spurious forces into the control mechanism and which directs the control port air itself to the exterior of a humidistat housing, thereby avoiding the necessity for constructing and installing parallel air supplies requiring increased compressor capability would be a distinct advance in the art.